Small internal combustion four cycle engines typically include components such as a cam gear and a pair of cam lobes mounted on a camshaft. The cam gear meshes with a crankgear mounted on a crankshaft, thereby rotating the camshaft in timed relation to the engine cycle. Each rotating cam lobe reciprocates a push rod, which in turn respectively act on a rocker arm to alternate an intake valve and an exhaust valve between open and closed positions. In the case of an overhead cam type engine, the rocker arms normally act directly between the cam lobes and the valves.
It is well known to form camshaft components from powdered metals. For example, the Powder Metallurgy Design Manual, published by the Metal Powder Industries Federation (MPIF) of Princeton, N.J. and hereby incorporated by reference, describes a variety of items, including camshaft components, which can be formed from powdered metal.
Various proposals have been made for attaching the powdered metal components to the camshaft. For instance, U.S. Pat. No. 3,962,772 to Haller discloses a composite machine element such as a gear or cam. A powdered metal preform is formed in a conventional briquetting die-set, and then fit on a knurled shaft in a die cavity. A plunger then compresses the preform to solidify it and interlock it with the shaft. Also, U.S. Pat. No. 4,969,262 to Hiraoka et al. discloses a method of making a camshaft in which the cam is composed of outer and inner powder layers. The green cam piece is fit on a steel shaft, and the green camshaft assembly is then sintered to bond the cam piece to the shaft.
In most camshaft assemblies, the cam lobe and the cam gear are indexed to rotate at the same speed. The lobe and gear are therefore usually fixed to the shaft, normally at some distance from each other. Because the cam lobe is subject to wear from contact with the followers, it is desirable for the outer contact surface of the lobe to be relatively hard. Conversely, it is desirable that the cam gear have ductile teeth which are relatively strong and flexible to facilitate their meshing with the teeth of the crankgear. For these reasons, it is difficult to form the lobe and gear as a unitary piece.